1. Field of the Invention
The invention relates to a valve pin bushing for the hot runner of an injection mold.
2. Description of Background Art
In injection molds the molding material usually comes from a feeder (manifold) and, changing direction by 90 degrees, flows into the channel of a valve pin bushing which coaxially contains a shutter pin controlled by an external actuator. The shutter pin by translating can open or close an orifice, commonly called “gate”, located at the end of a heated nozzle communicating with the valve pin bushing and from where the material passes in the mold cavity.
Generally the entry of the valve pin bushing is coaxial to the duct in the manifold and with axis orthogonal to that of the channel in which the shutter pin slides. With this geometry the incoming plastic material is forced to deviate by 90 degrees and bump against the shutter pin, thereby generating two zones with different flow velocity: high, in front of the entry, and low or zero in the opposite part behind the shutter pin. Resulting material feeding fronts are oval or elongated, which cause asymmetries in the fillings.
To uniform the velocity inside the nozzles, valve pin bushes have been developed with particular geometries. In EP0264725 the valve pin bushing has two channels that convey the flow coming from the manifold to diametrically opposite sides of the shutter pin. For difficulty of construction the valve pin bushing is divided into two parts, complicating the assembly and construction thereof. The next variant in EP0405007 consists of a single piece, but tests and simulations in fluid dynamics show that in homogeneity of the flow within the nozzle and during the filling of the cavity persists anyway.